9.7. Fill-in functions

Occasionally you will see functions like addTime written with a different interface (different arguments and return values). Instead of creating a new object every time addTime is called, we could require the caller to provide an “empty” object where addTime can store the result. Compare the following with the previous version:

void addTimeFill (const Time& t1, const Time& t2, Time& sum) {
  sum.hour = t1.hour + t2.hour;
  sum.minute = t1.minute + t2.minute;
  sum.second = t1.second + t2.second;

  if (sum.second >= 60.0) {
    sum.second -= 60.0;
    sum.minute += 1;
  if (sum.minute >= 60) {
    sum.minute -= 60;
    sum.hour += 1;

One advantage of this approach is that the caller has the option of reusing the same object repeatedly to perform a series of additions. This can be slightly more efficient, although it can be confusing enough to cause subtle errors. For the vast majority of programming, it is worth a spending a little run time to avoid a lot of debugging time.

The active code below uses the fill-in version of the addTime function. Feel free to modify the code!

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